I used what could be described as Unity's default save location, if you're on windows, I imagine that's in AppData/LocalLow/PA, at least it is for me. The files should be named Mystery of Castle Super Beast_SaveData1-5, along with Mystery of Castle Super Beast_ConstantData (The file that checks if you've gotten some endings, if you've saved in the first three slots, other things and also keeps count of how many saves you've made) Just deleting those files should clear progress and allow you to start over from scratch.